This invention relates to a charging device of a contact type, more specifically to a charging device having a flexible contact member to be in contact with an object to be charged.
Conventionally, most electrostatic copying apparatus use chargers including corona dischargers. These corona dischargers, however, are subject to several defects; hazards due to the use of high voltage, pollution of the atmosphere and equipment caused by ozone, and high manufacturing cost. As for the conventional charging methods using rollers or brushes, photosensitive materials would be damaged mechanically and electrically. Thus, none of these methods can be adapted to those photosensitive materials which are to be used thousands of times or more.
Recently, a charging device has been proposed which solves the aforesaid problems having a contact member which is formed of pliable material. The contact member has a predetermined electric resistance and contacts with an object to be charged. An electrode is electrically connected to the contact member and has an electric resistance lower than the predetermined electric resistance of the contact member. Means are provided for supplying voltage on the electrode to charge the object being charging. This is a contact-type charging device in which a DC voltage and an attenuating AC voltage are applied to the electrode which is disposed at the back of the contact member formed of a material such as a brush with a proper electric resistance. In charging operations repeated thousands of times or more, the use of such charging device should produce good results.
The resistance of the contact member varies with the changes of ambient conditions, especially humidity. Since the change of the resistance of the contact member has a bad influence upon the charging efficiency, it is necessary to keep the resistance of the contact member at a desired fixed value. In the proposed charging device, therefore, the change of the charging efficiency caused by the influence of humidity is prevented by continually heating the contact mamber by means of a heater to keep the contact member dry, or by providing a mechanism capable of charging the contact pressure of the contact member on a photosensitive layer as the object of charging according to humidity. With such arrangement, however, the charging mechanism is further complicated in construction, and it is necessary continually to supply power for heating and humidity sensing, resulting in an increase in running cost.
If the resistance of the contact member is reduced by humidity, furthermore, the amount of current flowing into the photosensitive material will increase possibly to shorten the life of the photosensitive material.